The number of wireless communication modalities sharing the same frequency band continues to increase. This means that simultaneous transmissions are more likely to interfere with each other, particularly if an interfering transmitter is close to an intended receiver. This decreases the reliability of the wireless communications.
Interference rejection has been used to protect against partial-band and partial-time interference. Two known methods can reject interference: erasure and clipping. For the erasure, the receiver detects whether a signal sample is corrupted by interference, and erases the sample with a zero. For the clipping, the sample is replaced with a neighboring uncorrupted sample.
Another approach uses message passing and models interference as equivalent Gaussian noise. The existence of interference is detected, and its variance is estimated. Then, the log-likelihood ratio (LLR) of received symbols can be determined based on the estimated interference variance. Soft-iterative decoding is conducted using the LLRs as input to channel decoder to resolve the interference.
OFDM networks are used for high data-rate transmission in multipath channels, e.g., networks according to the IEEE 802.11a and 802.11g (WiFi) standards. Those networks use a fast-Fourier transform (FFT) to convert inter-symbol-interference (ISI) time-domain channels into parallel frequency-domain channels. Thus, symbols are transmitted without ISI in the frequency domain. An OFDM symbol includes a set of data signals and a set of pilot signals, each on a different subcarrier. Known symbols are transmitted using the set of pilot signals to estimate the channels for the set of data signals.
Partial-band and partial-time interference (PBPTI) can corrupt the transmission of wideband OFDM signals. On the unlicensed radio spectrum, Bluetooth networks can coexist with OFDM networks. The frequency-hopping Bluetooth signals block the transmission of some subcarriers of wideband OFDM signals, thereby generating PBPTI for OFDM networks. The interference corrupts consecutive subcarriers and hops to different subcarriers over the transmission.
Most known methods require statistics of the channel and interference before any processing can be done on any portion of the received signal. For example, a hypothesis test can be used for interference detection. Also, when the channel statistics, e.g., the power-delay profile, are known, time-domain channel estimation method can be used. Compared to the frequency-domain channel estimation approaches, the time-domain method estimates a smaller number of unknown channel coefficients. Thus, it is more resilient to interference than the frequency-domain method. When the interference statistics are known, interference can be treated as noise with a known variance. Its log-likelihood ratio (LLR) can be determined, and soft iterative decoding can be used to recover the data.
When the channel and interference statistics are not known at the receiver, estimating all required parameters using prior art methods can be prohibitively complex. The invention solves this problem.
In addition to PBPTI, OFDM networks are vulnerable to fast-varying channel conditions. It is desired to provide OFDM networks and methods for joint wireless channel estimation and PBPTI detection with reduced interference.